1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to an apparatus and method for driving a lamp of a liquid crystal display device that may apply a scanning backlight driving to an external electrode fluorescent lamp.
2. Discussion of the Related Art
Liquid crystal display devices have numerous applications and this is an every-increasing trend due to characteristics such as lightness, thinness, low driving power consumption, and other factors. According to this trend, liquid crystal display devices may be used in office automation equipment, audio/video equipment, and other devices. The liquid crystal display device controls the amount of light transmitted in accordance with a signal applied to a plurality of control switches which may be arranged in a matrix shape, and thus a desired picture is displayed on a screen.
The liquid crystal display device is not a self luminous display device, rather it requires a separate light source such as a backlight.
Backlights may be classified into either a direct type or an edge type according to the location of a light source. The edge type backlight has a light source installed at the edge of one side of a liquid crystal display device, and irradiates light from the light source to a liquid crystal display panel through a light guide panel and a plurality of optical sheets. The direct type backlight has a plurality of light sources disposed right under the liquid crystal display device, and irradiates light from light sources to the liquid crystal display panel through a diffusion plate and a plurality of optical sheets.
Recently, in the direct type backlight, of which brightness, light uniformity and color purity are greater than those of the edge type backlight, is more often used in the case of an LCD TV.
FIG. 1 is a diagram representing a liquid crystal display device of the related art to which a direct type backlight may be applied.
Referring to FIG. 1, the liquid crystal display device of the related art includes a liquid crystal display panel 11 for displaying images and a backlight unit 10 for irradiating light to the liquid crystal display panel 11.
In the liquid crystal display panel 11, a plurality of data lines and a plurality of gate lines are arranged to cross each other (not shown), and liquid crystal cells are arranged in an active matrix type between upper and lower substrates (not shown). Further, in the liquid crystal display panel, there are formed pixel electrodes and a common electrode for applying an electric field to each of the liquid crystal cells (not shown). Thin film transistors, for switching a data voltage which is to be applied to the pixel electrode in response to a scan signal, may be formed at crossings or the data and gate lines. In such a liquid crystal display panel, gate driver integrated circuits and data driver integrated circuits may be electrically connected through a tape carrier package TCP.
The backlight unit 10 further may include a plurality of lamps 15, a bottom cover 12, a diffusion plate 13, and a plurality of optical sheets 14.
The lamps 15 emit light by an AC high voltage from an inverter (not shown) to generate light to a diffusion plate 13.
The bottom cover 12 may be made as a container structure wherein a plurality of lamps 15 may be housed, and a reflection plate may be formed in bottom and side surfaces of an inner space of the bottom cover 120.
The diffusion plate 13 may be assembled with the bottom cover 12. The diffusion plate 13 may include a plurality of beads and disperses light incident from the lamps 15 by use of the beads such that no brightness difference between locations of the lamps 15 and locations between the lamps in the display surface of the liquid crystal display panel 11 exists. The diffusion plate 13 may be structured where the beads are scattered in a medium having the same refractive index, so that light can not be condensed.
The optical sheets 14 may include one or more diffusion sheets and one or more prism sheets, and irradiate the incident light from the diffusion plate 13 to the entire liquid crystal display panel 11 and bend a path of light in a substantially vertical direction to the display surface, thereby condensing the light to the front surface of the display surface.
The lamp 15 may be a cold cathode fluorescent lamp CCFL, an external electrode fluorescent lamp EEFL, or a light emitting diode LED.
The cold cathode fluorescent lamp, as shown in related art FIG. 2, may have inverters I1 to In connected to lamps L1 to Ln respectively, and can be independently driven for each lamp L1 to Ln by each inverter I1 to In. Accordingly, it is possible to apply a scanning backlight driving where the lamps are sequentially turned on/off such that motion blurring is reduced.
FIGS. 3 and 4 are related art diagrams briefly representing a scanning backlight as functions or voltage, frame and time.
Referring to FIG. 3, if a drive voltage of a liquid crystal cell is about 30V in the previous frame and a drive voltage of the current frame is about 80V, the voltage slowly increases in a curve shape as illustrated in FIG. 3 until it reaches a desired value. In order to reduce heat generation caused by the backlight, to decrease motion blurring, and to increase efficiency of the backlight, the lamp is not continuously turned on for the entire time of the current frame, but the lamp is maintained in an off state for a fixed time from the time when the current frame starts and is turned on for the remaining time.
Referring to FIG. 4, the turn-on time of each lamp may be controlled in an order of the lamps according to FIG. 3. If a gate signal is sequentially supplied to each gate line in the liquid crystal display panel, the lamp corresponding to the gate line to which the gate signal is supplied, as a result, the lamps are sequentially turned on.
The external electrode fluorescent lamp realizes a high brightness. The external electrode fluorescent lamp has an electrode located at the outside which differs from the cold cathode fluorescent lamp where the electrode is located inside the lamp, thus the lamps are generally operated in parallel to reduce a voltage deviation between the lamps. Thus, the external electrode fluorescent lamp is highly energy efficient and has a long life span as well uniform brightness, thereby the external electrode fluorescent lamp is preferred.
However, the external electrode fluorescent lamp differs from the cold cathode fluorescent lamp, as shown in related art FIG. 5, and is driven by a parallel driving where an external electrode 22 connected to both ends of a lamp 21 with a common electrode 23 is disposed and an inverter 24 is connected to the common electrode 23, making it almost impossible to drive the lamp independently such that scanning backlight driving cannot be accomplished.